Page History

Creation of M-Climate

The ER- M-Climate is derived from a set of medium range re-forecasts created using the same calendar start dates over several years for data times either side of the time of the extended ensemble run itself.  The re-forecast runs are at the same resolution as the medium range run itself (currently 9km?) and run over the 15-day medium range ensemble period.   

Medium range re-forecasts for verification

There is some merit in examining the real-time performance of a seasonal forecasting system.  But   But the sample sizes created for one system are far too small to conclude anything about the its true performance levels of that system. Hence we use the re-forecasts.  .  Re-forecasts are used to increase the available data to produce a model climate.   The results of forecast system may be compared with this model climate.  

Re-forecasts are a fundamental component of all seasonal forecasting system; they have two applications:

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The large number of re-forecasts is justified because it is essential to minimize sampling noise in the M-climate tails for EFI and SOT computations.  

The M-climate is used in association with the ensemble forecast:

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• 2m temperature.
• soil temperature.
• sea-surface temperature.
• mean sea level pressure.
• precipitation.
• cloudiness. 

Different reference periods for M-Climate and ER-M-Climate

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• For shorter ranges, the priority is the best possible capture of the climatological distribution of the tails (e.g. for EFI and SOT).  This can be better achieved using a re-forecast span of 5 weeks (1980 re-forecast values).  
• For longer ranges, the priority is the correct representation of seasonal cycles.  This can be better achieved by using a span of 1 week (660 re-forecast values).  The tails should not be so prone to having a reduced sample size.

M-climate is updated twice a week, every Monday and Thursday, and is based on 00UTC runs only (there are no 12UTC re-forecast sets).  The new files start to be used from the 00UTC run the next day.

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If for the same lead-time, one compares, the M-climate quantile plots (e.g. for a Thursday 00UTC run), and a run 24hrs later, they will be slightly different.  This limitation of twice weekly updates to the M-climate can be significant.  It can be particularly evident in spring and autumn when mean temperatures are changing most rapidly day by day.

Different reference periods for M-Climate and ER-M-Climate

ECMWF uses different reference periods but essentially the same re-forecast runs to build the M-Climate and the ER-M-Climate.   The key difference is that those runs are grouped and used in different ways:  

• For shorter ranges, the priority is the best possible capture of the climatological distribution of the tails (e.g. for EFI and SOT).  This can be better achieved using a re-forecast span of 5 weeks (1980 re-forecast values).  
• For longer ranges, the priority is the correct representation of seasonal cycles.  This can be better achieved by using a span of 1 week (660 re-forecast values).  The tails should not be so prone to having a reduced sample size.

Note before Cy41r1 in spring 2015, the M-climate was constructed from only 500 re-forecasts was more prone to sampling errors and as a result.

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